The present invention relates to the field of wireless communication in general, and in particular, to a router for switched array antennas for high capacity wireless broadband networks.
Wireless communication at high frequencies in the range of 1 GHz to more than 100 GHz are used extensively for point-to-point (PP) and point-to-multi point (PMP) communication. For these high frequencies, three types of antennas are commonly used for spatial directional data transmission. Parabolic reflector antennas are used for a fixed narrow spatial direction of transmission. Sectorial horn antennas are used for fixed wide area transmission. Patch antennas are used for fixed direction transmission as well. Those antennas have fixed lobe patterns aligned towards transceivers located in a well defined spatial sector. Once the data link is defined the antennas transmit and receive data from those fixed directions, based on the MAC (Media Access Control) layer either in a circuit connection form, in broadcast form or in a polling form. In PP and PMP systems, the transceivers"" antennas at both sides of each link have to be aligned to face each other and the antennas"" alignment is usually done manually during the initial link commissioning. When setting up a PMP link, the antenna beams at both sites have to be aligned simultaneously towards each other to reach maximum received signal. In PMP systems, the base station often includes fixed sectorial antennas that are set initially to radiate in well defined sectors, e.g. four low gain antennas of 90 degrees that are positioned to cover 360 degrees. Thus, a subscriber""s antenna has a narrower spatial divergence to increase its gain, is aligned towards the base station location in azimuth and elevation until maximum reception is achieved. This alignment guarantees that the base station is also receiving maximum transmission signal via its large lower gain fixed sectorial antenna.
Data packets transmitted and received by the antennas are coming from the same directions. In the case of PMP system that uses FDM B frequency division multiplexing, or TDM B time division multiplexing, or other modulation technique, the base station can broadcast information dedicated to specific transceivers located in a sector. All other transceivers in the same sector will receive the data, decode it, but will ignore it once it is found that the data is not aimed for them. However, by sharing the sector among many transceivers, only a limited amount of data packets can be forwarded simultaneously among the transceivers when transmitting at the same frequency.
The process of alignment in both PP links or in the case of adding a new subscriber at a PMP system is done off-line prior to service activation and involves accurate mechanical adjustment while monitoring the received signal level. In DBS (Direct Broadcast Systemxe2x80x94a PMP using a satellite), antenna alignment is done in a similar way to terrestrial PMP system. At the subscriber location, the antenna is aligned towards a geostationary satellite until a good signal is detected, and then it is fixed mechanically towards that direction. In all of the above-described cases, the antenna""s aperture is aligned mechanically towards the broadcasting source or towards each other before establishing the communication link and starting the service. Based on the received signal level, the direction is mechanically adjusted, sometimes by motor driven antenna, and fixed to the specified direction of maximum reception and transmission.
Few techniques are used to route or direct data towards different transmission directions. The most common is to locate a base station with multiple transceivers, each one with its own separate antenna, where each antenna covers a different sector. The base station MAC layer switches the data at baseband to the transmitter, which covers the sector that contains the subscriber transceiver site where the data packets are aimed. At a PMP base station, typical sectorial antennas such as horn antennas are designed to cover fixed 90, 45, 30 or 15 degree lobes in the horizontal plane and about 7 degrees in the vertical plane. The subscriber antenna, on the other hand, is designed with much narrower beam sensitivity, i.e. higher gain, with similar divergence in horizontal and vertical planes, usually less than 7 degrees. Horn antennas, lens corrected horns and parabolic antennas are commonly used for the subscriber transceiver. Other PMP systems use a subscriber radio with an antenna that receives the down-stream data from the base station in one polarization, say horizontal, and transmits upstream in a perpendicular polarization, say vertical, towards the base station, thus increasing network capacity. In all of the above cases, the spatial capacity in a sector is fixed by the alignment of the antennas.
Phased array antennas allow beam steering by controlling the phase of each antenna element relative to phase of the other elements thus allows beam steering. Those antennas are complicated to control in a very short duration imposed by the burst nature of the packets of data. Thus, phased array antennas are currently used only in some advanced cellular base stations to establish circuit connections for relatively long duration data transmissions, such as in circuit oriented networks where the duration of voice conversation is relatively much longer than packets of data. Phased array antennas are used primarily at low frequencies, typically less than 2.5 GHz, to get high directionality in a multireflections environment. The complexity, high cost and high loss of components, namely phase shifters, at high frequencies prevent use for mass commercial applications.
A simple solution for switching data packets towards different transceivers at different directions is by fast switching the final output energy between different sectorial antennas located in different angles, in say the horizontal plane, thus covering a large field of view. This configuration, however, demands a multiplicity of antennas, each one aimed in a different direction with a multiplicity of transmitters and connection lines to feed those antennas. RF energy needs to be switched and then transported, via long waveguides or coax, to each antenna. The distance from the switches to the antennas creates large signal attenuation, which increases at higher frequencies, and demands increased antenna structural dimensions, cost, and can be environmentally objectionable. Thus, an objective is to control a very fast switch for millimeter waves using a high frequency switched antenna array, with the switch located in close proximity to the antenna array. This is needed to allow high bit rate packets modulating high frequency RF to be efficiently switched towards different transceivers in different spatial directions.
An example of spatial routing of data packets in the space between arbitrarily distributed wireless nodes is described by Berger at el. in U.S. patent application Ser. No. 09/187,665 entitled xe2x80x9cBroadband Wireless Mesh Topology Networksxe2x80x9d, filed Nov. 5, 1998, incorporated by reference herein. The wireless network nodes are designed to select a transmission direction and a receive direction based on the routing address of the data packets to be sent and/or received. The selection of a transmission or receiving direction is done instantaneously to accommodate short bursts of data packets arriving from nodes located at different directions or transmitted towards nodes located at different directions, as defined by the scheduler of the MAC layer of the network nodes, as explained in the prior applications. A communication protocol that is designed to support the scheduling of spatially routed packets between network nodes in any generic topology such as mesh, tree and branch and PMP, is described by Aaronson at el. in U.S. patent application Ser. No. 09/328,105 entitled xe2x80x9cCommunication Protocol for Packet Data Particularly in Mesh Topology Wireless Networksxe2x80x9d, filed Jun. 8, 1999, incorporated by reference herein. The description of the media access control (MAC) layer is particularly pertinent.
The presently disclosed spatially switched router (SSR) describes a way of designing a data packet switching and routing apparatus capable of switching data packets and transmitting them spatially between wireless network nodes. The MAC layer defines, in real time, the direction and time of the RF switching, thus directing data packets based on the packets routes, destination and the network""s node spatial location. The prior applications explain that RF switching is established by schedules, held at each node whereby packets are directed and received from specified, spatially separated, nodes at appointed times. This MAC protocol is assumed in the present invention such that transmission and reception timing and the corresponding desired direction of transmission or reception are known in advance. However, other packet protocols can be used with address decoding and routing information obtained by decoding of the packets.
The SSR apparatus enables the switching of transmitted and received data packets from one node to other neighboring nodes and from multiple nodes located at different direction and distances to other nodes in their surroundings. Fast switching is accomplished by applying fast, in the range of few nanoseconds to a few microseconds, control signals to a series of microwave switches, synchronously with the data packet transmission and reception timing and synchronously with the direction of transmission and reception. The fast RF switches are designed in a configuration that delivers large isolation between the receiver and transmitter input ports and minimizing the RF losses. The design allows close proximity of the switches to the output feeding ports to reduce coupling and transmission losses especially important at very high frequencies ( greater than 20 GHz). An nxc3x97m switch assembly (n=number of input ports, m=number of output ports) is designed based on a series of custom made 2xc3x974 integrated RF switches made of GaAs integrated circuits (MMIC), designed for the very high microwave frequencies ( greater than 20 GHz) and a switching array assembly structure closely coupled to the focusing and collimating antenna structure.
A principal feature of the current spatially switched router apparatus is its wireless spatial packet routing and switching capability to form a xe2x80x9cconnection-lessxe2x80x9d communication link between a multiplicity of dispersed nodes in a mesh topology network or any other derivative of a mesh topology network such as tree and branch and/or PMP. At the very high microwave frequencies, the system may require a line-of-sight (LOS) between the communicating nodes. The spatial transmission of data packets, such as internet protocol (IP) packets, towards specific directions of the destination nodes allows multiple nodes to transmit at the same time, at the same frequency band and in the same area with minimum mutual interference. This synchronized mesh network increases the available capacity of the network dramatically relative to the common xe2x80x9cconnection orientedxe2x80x9d networks, used in many PMP systems. In those PMP systems, the bandwidth at certain sectors is defined up front by the antenna""s fixed illumination pattern. The spatially switched router apparatus of the present invention can perform fast route diversity and fast load balancing, taking full advantage of the bursty nature of the IP data packets traffic.
The present invention is optimized for the very high radio frequencies, such as the FCC assigned LMDS (Local Multipoint Distribution Systems) spectrum, 27 GHz to 31 GHz, and other spectra that are assigned to operators on a regional basis. Those frequencies bands allow large amounts of data distributed at such frequencies, as 10.5 GHz (UK, Latin America) 24.5-26.5 ; (Europe) 38 GHz-40 GHz (U.S.) etc. At those frequencies, the attenuation of transmission lines is very high. Thus, the current design is made of a very compact switch array matrix that is closely coupled to multiple feeding ports, which are designed to feed multiple, focusing and collimating ports of a beam forming optics apparatus operating at radio frequencies. One of the beam forming apparatuses described in this invention comprises a known multi-layer, graded-index, cylindrical lens that forms a one dimensional, say horizontal focusing device, wherein the other dimension, say vertical, divergence is defined by the aperture size of the feeding port horn. This apparatus design allows the formation of beams with different divergences in the horizontal plane, where the horizontal plane is the switching plane, and the vertical plane.
In a different beam forming apparatus of the present invention, the feeding ports feed a multi-layer graded-index spherical lens, such as an RF Luneberg lens, to form beams with similar divergence in the horizontal (switching) plane, and the vertical plane. In both devices, beam switching can cover angles in excess of 120 degrees with very high gain and collection efficiency from different directions inside the sector. The packets of data modulating the RF carrier are switched to focal points, where beams from different focal points are collimated to destination directions. All the beams share the same lens, and use an overlapping aperture, of the cylindrical or spherical lens, thus significantly decreasing the size of the wireless node antenna. The smaller size allows lower losses of RF energy coupled through the antenna, lower weight and minimal intrusion in the environment.